Thanks for reaching out. Let's check on this further.
A few questions: what type of Chromecast device do you have? Is this an initial set up or working previously? If working previously, are there any recent changes in your Wi-Fi network? Please try to reboot your Chromecast, Wi-Fi router, and mobile phone. Using a different mobile device and manually connecting to the Chromecast SSID might also help.
Here's how you can connect manually to the Chromecast SSID: Go to the phone settings > Open WiFi and connect to the Chromecast SSID (Example: Chromecast1234, Living Room TV, Bedroom TV, etc.) > Go to the Google Home app and try to set up again.